A wide variety of automobile components have traditionally been formed from steel alloys and, more recently, from aluminum alloys. In many instances the corrosion resistance of these components is bolstered to ensure product longevity; especially since the component is likely to encounter harsh environments. For example, automobile body frame and panel structures constructed from joined steel and aluminum alloy parts are routinely provided with a variety of coating layers to protect the underlying metal alloys against corrosion-advancing substances such as air, water, and salt. These layers may also be designed to ensure that later-applied paint coatings adequately adhere to the body structure's surface. And customarily the automotive industry has applied these corrosion-resistant coatings by progressively advancing automobile body structures and related panels (sometimes called a body in white) through a protective coating and painting line that includes a series of procedures such as, among others, a phosphating and electrocoating (e-coating) stage.
In recent years, however, the incorporation of magnesium alloys into automobile components has been the subject of considerable investigation and development due to those materials' relatively low density as compared to other structural metals. The magnesium alloys may be used in pillars, frames and other body members, and the alloys may be used in sheet metal form as inner or outer body panels or the like. The magnesium alloy components may be attached to complementary aluminum alloy or ferrous alloy parts by welding, bolting, riveting, hemming, or other suitable joining process. But unfortunately, unlike steel and aluminum alloys, magnesium-based metals are not particularly well suited for exposure to conventional phosphating and e-coating procedures; mostly because magnesium surfaces are highly active when exposed to neutral or acidic aqueous liquids. A number of issues have thus arisen when attempting to progress a multi-metal automobile component that includes magnesium alloy parts through a conventional automobile paint-line. On one hand, it is desirable to advance the component through the paint-line so as to adequately treat any steel or aluminum alloy parts against corrosion prior to painting. But on the other hand the magnesium alloy parts deteriorate in the process.
In light of these and other issues a component's magnesium alloy parts may be protected against corrosion by alternative practices, such as spark-anodizing (micro-arcs, plasma electrolytic oxidation, etc). These practices sometimes require the component to be redirected off-line and separately treated from the conventional automobile paint-line. And they can also necessitate selective and precise treatment of only the specific magnesium alloy parts of the component; an option that is, practically speaking, rather difficult. These alternative practices can thus become tedious, inefficient and expensive. What is therefore needed is an efficient and easily operated technique to protect components made wholly or partly of magnesium and magnesium alloy parts that can potentially take advantage of the automobile paint-lines and procedures currently being utilized.